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CSMA/CA Based Protocol

A Distributed Access Protocol Proposal

Supporting Time Bounded Services

By: Wim Diepstraten

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COMPARE ALOHA, CSMA and WAVELAN CSMA/CA

- WAVElAN CSMA/CA Throughput is 87"- or the 2 Mbps raw bil rale.

- WAVELAN CSMA/CA is stable at high loads.

- The Delay is only calculated [or Lhose packets that get throug:l.

Figures include the MAC overilead.

Load generated by 7 Stations.

SublDlSSlon Page 26 ----wiiiI Diepstriten

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May 1992 Doc: IEEE P802.1l-92/S1

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Throughput versus Buffered Load Curve

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THROUGHPUT : - WAVELAN CSMA/CA - I-persistenl CSMA - - ALOHA

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COMPARE ALOHA, CSMA and WAVELAN CSMA/CA

- WAVELAN CSMA/CA Throughpul Is 87"- or lhe 2 Mbps raw bit rille.

- WAVELAN CSMA/CA is stable at high loads,

The Delay is only calculaled [or ltlOse packets that gel through.

- Figures include the MAC overhead.

- Load generated by 7 Stations.

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Throughput versus Buffered Load Curve

60:0: Short Packets (64 Byll!sl, 40:0: Long Packets (576 Byll!s)

THROUGHPUT : - IYAVElAN CSIoIA/CA - SMA/CA + Ack

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Fig: 3 Buffered Load (KByle)

COMPARE CSMA/CA. CSMA/CA + Ack and 4-WAY LBT

- The Delay is only calculaLed for those packets that get through .

- For CSMA/CA the lost packets are not recovered so it is not included in the delay figure.

- Figures include the MAC overhead .

SubmisslOD Page 29 Wlm DiepstnlleD

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May 1992 Doc: IEEE P802.11-92/S1

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Fig: 2 Buffered Load (KByle)

COMPARE CSMA/CA. CSMA/CA + Ack and 4-WAY LBT

- WAVELAN CSMA/CA Throuahput b 87:0: 0' the 2 IoIbps raw bit rale

WAVElAN CSMA/CA Is stable at high loads.

- The Delay is only calculated for those packets that get through .

Figures include the MAC overhead.

Load generated by 7 Stations.

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THROUGHPUT : -- WAVELAN CSMA/CA - CSMA/CA + Ack

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Fig: 4 Number or Stations

COMPARE CSMA/CA. CSMA/CA + Ack and 4-WAY LBT

- Figures include the MAC overhead.

= Throughput remains slable ror many simullaneous stations accessing the medium.

- For CSMA/CA lost packets are not recovered at MAC level

SubmiSSion Page 30 Wlm Dlepstraten

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IMay 1993 Doc: IEEE PS02.11·93170a I

Wireless Network Architecture

Wireless Network Architecture

* Infrastructure mode "Base Station oriented" . Default all traffic goes via the AP.

Protocol Proposal Slide 1

IMay 1993 Doc: IEEE PS02.11.93170al

Supported Services: * Asynchronous Data Service:

Short response time with high instantaneous Throughput. Suitable for Bursty traffic.

* Time Bounded Service (Optional): Time Bounded Service dimensioned for Voice. Allows mixed Voice/Data. Requires sufficient BSS isolation, (TDMA type access protocols have similar requirements).

Protocol Proposal Slide 2 Page 6

I May 1993 Doc: IEEE PS02.11-93nOal

Basic Access Protocol: * Use Distributed Access Protocol for efficient

medium sharing.

*

*

Robust for interference. CSMA/CA + Ack for unicast frames. With MAC level recovery CSMA/CA for Broadcast frames.

Supports Ad-Hoc operation seamlessly

Protocol Proposal Slide 3

IMay 1993 Doc: IEEE P80z.11-93nOal

CSMA/CA explained:

* *

Fne _ when medium AJFS is free longer then AIFS r::~-~~~~~

AIFS

Busy Medium Next Frame

Dtrer ACC'tU Select Slot and Sense Medium -Ilain

CSMd/CA dcce$'.\' Methodology with IsochronQus cauability

Reduce collision probability where mostly needed.

Implement different priority levels. (to allow immediate Ack and Isochronous coexistence)

Protocol Proposal Slide 4 Page 7

IMay 1993 Doc: IEEE PS02.11.93nOal

CSMA/CA+Ack Access Protocol AckiFS

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Now. Aloo &bo InIIIDIIlWotthe ___ Backoft'· Window "'-_ID_ ... _1IotG&, AD'S willi tho _ r ........ II ..tIIc:ItMI, IIr1' .. .-..... .. 1II ............... oIAIFS. III /I IIJ Next F rame

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CSMAICA + Ack Asynchronous Access Methodoio:y

* Exponential Backoff with Access Retry limit. * Retransmission after Random delay when no Ack

received.

Protocol Proposal Slide 5

IMay 1993 Doc: IEEE PS02.11.93nOal

Async service Characteristics:

*

*

* *

*

Short response time, and high throughput efficiency similar to 802.3 CSMA/CD.

Lost packets due to collisions and/or interference are recovered at the MAC level.

Stable under high Load conditions.

Etlicient Medium sharing without added control overhead.

Includes Coexistence Provisions for Time Bounded Services.

Protocol Proposal PageS

Slide 6

IMay 1993 Doc: IEEE P802.11.93nOal

AsynclTime Bounded model:

HIe krriq Herld

* Data, Voice or mixed implementations possible.

* Time Bounded Service uses Point Coordination Function.

* Time Bounded capability does not burden the Async service implementation.

Protocol Proposal Slide 7

IMay 1993 Doc: IEEE P802.11.93nOal

Time Bounded Characteristics: * Built on Asynchronous Access method.

* Uses CSMA/CA + (Ack) with highest priority. With limited Asynchronous recovery.

* Dimensioned to support mixed Voice/Data.

* Video support possible at higher PHY rates. * Isochronous Framing Period is PHY speed dependent

IFP= 20-25 msec for a 2 Mbps PHY.

* Unused reserved Isochronous Bandwidth can be used for Asynchronous traffic.

Protocol Proposal Page 9

Slide 8

IMay 1993 Doe: IEEE PS02.11.93170a1

Time Bounded Characteristics: * Can support different PRY speeds (1-20 Mbps)

* Support variable packet size on a per frame basis, without control overhead.

Can take full advantage of "Talk Spurt" characteristics of Voice. Allows flexible congestion control.

* Includes provisions for Power Consumption Management.

* Includes basic re-association provisions.

Protocol Proposal Slide 9

IMay 1993 Doc: IEEE PS02.11·93nOa I

Time Bounded Services (Cant.): ToUI_

- ---++--++ --Beacon

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Beacon

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Isochronous Access Protocol * Time Synchronization assumed by regular Beacon.

Protocol Proposal Slide 10 Page 10

IMay 1993 Doc: IEEE PS02.1l.93nOal

Performance Example: * Assumption: 32Kbps ADPCM Voice

15 Byte MAC + Wavelan PRY Overhead

* Example: @ 2 Mbps Modem speed:

Voice only: Voice/Data 576 Byte: VoicelData 1500 Byte: VoicelData 1500 Byte:

Async Data throughput: Async Data throughput: Async Data throughput:

Protocol Proposal

IMay 1993

28 FDX (using talkspurt) 23 FDX (using taIkspurt) 18 FDX (using taIkspurt) 9 FDX (100% load)

> 75 KByte/sec (assuming max Isoc load)

>130 KByte/sec (assuming 9 FDX Talkspurt channels)

>200 KByte/sec (no !soc connection active)

Slide 11

Doc: IEEE PS02.U.93nOal

Conclusion:

* CSMA/CA + Ack is an efficient distributed access method with good medium sharing characteristics.

* A very flexible Time Bounded Capability has been demonstrated, built on top of CSMAICA.

* Added advantage over TDMA is its frame size flexability and optimum Asynchronous and Time Bounded sharing characteristic.

Protocol Proposal Slide 12 Page 11

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I May 1993 Doc: IEEE-P802:ii-93170-; I

Service required from the PHY

* Fast CS function with approx. 10 dB more sensitivity than the acceptable Data quality level.

* Fast Rx-to-Tx Turnaround time. Protocol Proposal Slide 13